The present invention generally relates to handling of occurrences of bubbles on the thermometer code bus of a flash A/D converter or an A/D converter stage of a multi-stage A/D converter.
Flash A/D converters include a set of comparators that simultaneously convert an analog signal into thermometer code, which is converted into binary code. This conversion relies on detection of the location of the 1-to-0 transition in the thermometer code. However, due to component imperfections in the converter, there may in fact be several such transitions in the thermometer code. This is called a bubble or sparkle and leads to ambiguities in the conversion. References [1] S. Padoan, A. Boni, C. Morandi and R Venturi, xe2x80x9cA novel coding scheme for the ROM of parallel ADCs, featuring reduced conversion noise in the case of single bubbles in the thermometer codexe2x80x9d, IEEE International Conference on Electronics, Circuits and Systems, 1998; [2] M. Sugawa, H. Yoshida, M. Mitsuishi, S. Nakamura, S. Nakaigawa, Y. Kunisaki and H. Suzuki, xe2x80x9cA 2.5V 100 MS/s 8bit ADC using Pre Linearization Input Buffer and Level Up DAC/Subtractorxe2x80x9d, IEEE Symposium on VLSI Circuits Digest of Technical Papers, 1998; [3] Jy-Der David Tai, xe2x80x9cBubble evaporation circuit for eliminating bubble errors in thermal code and its methodxe2x80x9d, U.S. Pat. No. 6,038,662 (as signed to Powerchip Semiconductor corp.); and [4] Andrew G. F. Dingwall, xe2x80x9cDecoder error prevention apparatus for use in flash analog-to-digital convertersxe2x80x9d, U.S. Pat. 5,023,613 (assigned to Harris Semiconductor Patents, Inc.); describe different algorithms for handling bubbles in the thermometer code. The purpose of these algorithms is to avoid catastrophic conversion errors in the thermometer-to-binary decoder due to the existence of a bubble. However, although these methods are bubble tolerant, they still produce large errors and they do not attempt to eliminate generation of future bubbles.
An object of the present invention is a calibration method and apparatus for a flash A/D converter or an A/D sub-converter in an A/D converter stage that reduce or eliminate occurrences of thermometer code bubbles.
This object can be achieved by a calibration method handling occurrences of thermometer code bubbles in an A/D sub-converter in an A/D converter stage, including the steps of:
detecting two A/D sub-converter comparators causing a bubble;
increasing the threshold of the bubble causing comparator having the lowest threshold by a first predetermined voltage; and
decreasing the threshold of the bubble causing comparator having the highest threshold by a second predetermined voltage.
The first and second voltages can be fractions of the A/D sub-converter quantization step. The first voltage can be equal to the second voltage. The method may further include the steps of:
determining the last A/D sub-converter comparator having a threshold that is smaller than an analog A/D sub-converter input signal;
increasing the threshold of the determined comparator by a third predetermined voltage if a residual signal from the A/D converter stage falls below a predetermined minimum level; and
decreasing the threshold of the first A/D sub-converter comparator having a threshold that is larger than the analog A/D subconverter input signal by a fourth predetermined voltage if the residual signal exceeds a predetermined maximum level. The third and fourth voltages can be fractions of the A/D sub-converter quantization step. The third voltage can be equal to the fourth voltage. The thresholds can be modified by modifying comparator offsets.
The object can also be achieved by a calibration apparatus handling occurrences of thermometer code bubbles in an A/D sub-converter in an A/D converter stage, comprising: means for detecting two A/D sub-converter comparators causing a bubble; means for increasing the threshold of the bubble causing comparator having the lowest threshold by a first predetermined voltage; and means for decreasing the threshold of the bubble causing comparator having the highest threshold by a second predetermined voltage.
The apparatus may further comprise means for determining the last A/D sub-converter comparator having a threshold that is smaller than an analog A/D sub-converter input signal; means for increasing the threshold of the determined comparator by a third predetermined voltage if a residual signal from the A/D converter stage falls below a predetermined minimum level; and means for decreasing the threshold of the first A/D sub-converter comparator having a threshold that is larger than the analog A/D sub-converter input signal by a fourth predetermined voltage if the residual signal exceeds a predetermined maximum level. The apparatus may further comprise means for modifying the thresholds by modifying comparator offsets.
The object can also be achieved by a multi-stage A/D converter including a calibration apparatus handling occurrences of thermometer code bubbles in an A/D sub-converter in at least one A/D converter stage, the calibration apparatus comprising means for detecting two A/D sub-converter comparators causing a bubble, means for increasing the threshold of the bubble causing comparator having the lowest threshold by a first predetermined voltage, and means for decreasing the threshold of the bubble causing comparator having the highest threshold by a second predetermined voltage.
The calibration apparatus may further comprise means for determining the last A/D subconverter comparator having a threshold that is smaller than an analog A/D sub-converter input signal, means for increasing the threshold of the determined, comparator by a third predetermined voltage if a residual signal from the A/D converter stage falls below a predetermined minimum level, and means for decreasing the threshold of the first A/D sub-converter comparator having a threshold that is larger than the analog A/D sub-converter input signal by a fourth predetermined voltage if the residual signal exceeds a predetermined maximum level. The multi-stage A/D converter may further comprise means for modifying the thresholds by modifying comparator offsets.
The object can also be achieved by a flash A/D converter handling occurrences of thermometer code bubbles, comprising means for detecting two A/D converter comparators causing a bubble, means for increasing the threshold of the bubble causing comparator having the lowest threshold by a first predetermined voltage, and means for decreasing the threshold of the bubble causing comparator having the highest threshold by a second predetermined voltage.
The flash A/D converter may further comprise means for modifying the thresholds by modifying comparator offsets.
Briefly, the present invention identifies the A/D converter comparators that are responsible for a detected bubble and adjusts their thresholds to reduce or eliminate the threshold crossover that caused the bubble. An advantage with this technique is that the A/D converter will eventually be bubble-free. This is an especially attractive feature for residual based correction methods in multi-stage A/D converters, such as pipeline, cyclic or sub-ranging A/D converters.